Pulse echo system for distance measuring



Patented Nov. ,30,1948

PULs sono ySYSTEM Fon DISTANCE Massimino PrafullaKumar Chatterjee. and Leslie Wilfred Houghton, London W. C. Z, England, assignors to Standard Telephones and Cables Limited, London. England, a British company Application December 10, 19422,y Serial No. 468,-57'0A f In Great Britain March 20, 1.942

i Claims.

This invention relates to methods of locating obstacles, such as aircraft, by means' of reiiected electromagnetic waves.

It provides a convenient means for indicating v immediately on a direct current instrument the distance of the object, and the instrumentmay be calibrated in miles or kilometres if desired; and the arrangements can be easily adapted to give an alarm when an aeroplane, for example, comes within a predetermined distance. A continuous indication of the distance is sivenfor the whole time that the obstacle is within the predeter mined rance. i

According to the invention, there is provided an obstacle locating system comprising means for radiating trains of regularly repeated pulses of electromagnetic waves and means for receiving the pulses after reflection from the obstacle, in which the radiated pulses are derived from a Sawtooth relaxation oscillator, characterised in this, that means is provided for cousinsP the reected pulses to synchronise a saw-tooth `relaxation oscillator.

The invention will be more clearly understood from the following detailed description. which re fers to the accompanying drawing in which' Figs. i and 5 show bloclr schematic circuit diagrams of two embodiments;

Figs. 2, 3, 4 and 6 are diagrams used for er i `planatcry purposes; and

Fig. 7 is a circuit diagram of a sawtooth oscillator.

@ne embodiment of the invention is shown in the form of a block schematic circuit in Fig. l. A saw-tooth relaxation oscillator' i o conven tional type controls a pulse generator 2, which delivers the pulses to the radio transmitter 3 and transmitting aerial i. The pulses reflected from the object are picked up on the receiving aerial il, and demodulated in the usual way in a radio receiver t, and are then ampliiied, and if necessary reshaped 'in the pulse amplifier l, and are returned to the relaxation oscillator i to control the saw-tooth Waves generated. The opera"- tion of the circuit will be explained with reference to Fig. 2, in which the full line curve c represents the voltage of the saw-tootll oscillations plotted against time, assuming that no reflected pulses are being received. The period ci these oscillations is T seconds, the'oorresponding frequency being per second V of 'the apparatus, and although The pulse generator 2 is designed according to A known principles to emit a short pulse at the moment of each ily-back stroke of the oscillations, that is, at the times :c in Fis. 2. The time T is chosen to be equal to twice the time necessary for a pulse to reach the most distant object the apparatus is designed to locate. Now suppose an object cornes within the range of measurement, then reflected pulses will loe received on the aerial t some time t after they were radiated, t being less than T. Consider s. pulse radiated at zero time 0, Fig. 2. The corresponding reflected pulse will return after time t at y and the circuit is designed so that it causes the ily-back stroke to be produced by the'oscillator i at that moment. The next pulse is at the same time emitted from the generator t, and this pulse comes back after again a time t and produces another flic-back stroke, and so on. Thus the oscillator now generstes the wave shown by the dotted line b in Fig. 2, the `period being t and the frequency l/t. Thus, the frequency of the oscillations generated by the oscillator l determines the distance of the object. To measure this frequency, the output of the oscillator i is applied to the device i3 which comprises a filter for separating out the fundamental frequency l/t, and s. discriminator circuit of some known type giving, for example, a continun ous output voltage proportional to the frequency. This may be connected to s. suitable voltrneter t), the indications of which can clearly be calibrated in terms ci the distance ci the object, if desired.

It is to be noted that direct transmission from aerial d to aerial t does not matter, because transm v inltted and reflected pulses always occur at the same time, although each reflected pulse correspends to the previous transmitted pulse. There fore the combined effect of the transmitted and reflected noise synchronize the oscillator i and thus it is not actually necessary to have two aerialszthe same one may be used for both purposes.

Reflected pulses will, ci course, be received from objects while they are bevond Vthe operating range i they may be of small amplitude, they may still be able to conu trol the saw-toctl1 oscillator i. Since the reflected pulses will return after a time greater than the period T of the oscillator i, extra pulses will be generated, but the irregular train so obtained will not be confused with the regular train produced by objects which are within range.

If desired, however', impulses reilected from objects outside the range of the apparatus or unwanted impulses reflected from other objects nearby may be prevented from producing any effect in the following way. Y

It is well known to use a gas discharge triode or similar device to operate a saw-tooth oscillator. Buch devices are described for example on 'page 456 of Television," by Zworykin and Morton, published 1940 .by John Wiley 8: Sons, Inc., New York. With the control grid biassed with a given negative voltage, the valve becomes conducting when the anode reaches a certain positive potential, when it discharges the condenser in the relaxation circuit. This potential is inversely related to the grid voltage; that is, when the grid voltage increases (becomes less negative), the anode firing potential decreases. The incoming pulses synchronise the oscillator by suddenly increasing the grid voltage, causing the valve to fire, and the grid voltage change necessary to nre the valve will be smaller when the anode voltage is high, than when it is low. This effect is Shown in Fig. 3 by the line c which relates the anode voltage Va of the gas discharge valve to the corresponding grid voltage Vy ynecessary to cause the valve to lire. The values of Vg are referred to a zero represented by the normal grid bias voltage chosen for the circuit, and they give the amount by which the grid voltage must be changed iny order to fire the Valve when the instantaneous anode potential is given by the corresponding abscissa. When the valve is generating saw-tooth oscillations,I the anode voltage is constantly varying so that the grid voltage change necessary to fire the'valve at any point in the charging stroke will vary in a corresponding manner. This is shown in Fig. i where the curve a shows on a larger scale the anode voltage during the saw-tooth oscillations, taken from Fig, 2. The line Qq shows the corresponding change in the grid voltage Vg necessary to produce a ilyback stroke at the time indicated by the abscissa, as obtained from Fig. 3. At the time x corresponding to the normal ily-back stroke, the firing grid voltage change is, of course, zero.

The method of eliminating the effect oi reilected pulses which are outside the range of the measuring device consists in applying to the grid of the gas discharge valve a variable bias which is determined by the frequency of the saw-tooth oscillations. This bias is arranged to be just enough to prevent the valve from being fired by the pulses from the amplifier 1 of Fig. 1 at any time except just before the normal firing time at cc, Figs. 2 and 4, or after a period onlyslightly different from the period of the previous cycle. Thus in Fig. 4, if the reflected pulses arrive after a time t which is less than T, they will have to overcome a small bias QM, which they will be able to du.` The frequency will be changed to l/t and it is arranged that the grid bias is thereby changed so that the line Qq moves down to Mm passing through the point M Where the vertical line QM cuts the time axis.

If a reflected pulse should arrive from a more distant obstacle after a time t1 which is slightly greater than T, the valve will have already fired at the normal time T, only shortly before, and the grid bias PN which must be overcome by the incoming pulse is now large. The pulse will moreover be of smaller amplitude owing to the greater distance of the obstacle, and the circuit conditions can be chosen so that it will be unable to fire the valve again. When the obstacle moves within range, the reflected pulse will be able to take control as soon as the time t of arrival becomes equal to or less than T, and the grid bias arranged as in Fig. 1.

4 will be thereafter varied as the time t becomes smaller, so as to maintain the intersection M of QM and Mm practically on the time axis. The

reflected pulse will retain control, while the ob= ject moves about, until t becomes greater than T. The necessary variable grid bias is produced by obtaining from the discriminator 8 in Fig. 1 a' anode resistance il and charging condenser it.

A standing bias is provided for in the usual manner by the potentiometer resistance i 9 and 2u connected between H. T. positive and earth which.

is also the H. T. negative with condenser 2i acting as a cathode Dy-pass condenser. The grid circuit is completed by resistances 22 and 23 in series.

The saw-tooth wave form' whichis developed across condenser i8 is fedV to the pulse generator 2 of Fig. 1. Reflected pulses shaped by the amplifier l' are fed in known manner via condenser 2d to the grid of valve i6 where if the'standing bles voltage is of the correct value, each puise will trigger valve I6 so initiating another ilyback stroke and hence a radiated pulse as already explained.

A voltage from the device i5, Fig. 1, is developed across resistance 23 and will vary the negative bias applied to valve I8 by the resistance 2U. Hence the arrangement is such that when the oscillator valve i6 is running normally without a synchronizing signal at its lowest frequency, it will have a maximum negative bias so that the amplitude of a pulse supplied by amplifier l, Fig. l, over condenser 2li will only be suilicient to trigger valve i6 if it occurs .iust before the normal iiringr time, as has been explained with reference f to Fig. 4. A located obstacle approaching the locator will, of course. cause the saw-tooth frequency to increase causing a corresponding reduction of the negative bias voltage.

With the arrangement just described. obstacles outside the range of the apparatus will not produce any indication.

Fig. 5 shows another arrangement according -to the invention. A saw-tooth oscillator l, pulse generator 2, radio transmitter 3 and aerial 6 are The receiving aerial 5 and receiver 6 are also as before, but the reflected pulses after demodulation in 6 are applied to synchronise another saw-tooth oscillator It. An inverting device. il, which may comprise an amplifying valve, inverts the saw-tooth waves generated by l, and applies them to the mixer i2 to be combined with the waves at the output of oscillator I0. Assuming that the waves generated by the oscillator I are as shown by the curve a,

in Fig. 2, the waves generated by oscillator i@ will be as shown by curve ein Fig. 6 they will be like those of Fig. 2, but displaced by a time t. Curve f of Fig. 6 shows the waves of Fig. 2 after inversion by the device il. After these two waves have been mixed together in i2 they will appear as shown in Fig. 6 at g. They are then applied to a rectiiier i3 which converts them into uni` directional pulses having the same duration as before, equal to the time interval t between the original and reected pulses, the pulses being reassura heatedat intervals of T. It these unidirectional pulses be now applied to an ordinary direct current instrument, such as a voltmeter i4, the reading obtained will be t.V/T, where V is the voltage corresponding to the height of the unidirectional pulses. Thus since T is known, the voltmeter reading gives t, and hence the distance of the object.

Itgnay' be mentioned that if desired. the sawtooth waves from the oscillator. Ill could be invertedinstead of those from oscillator i before mixing in the device i2. The resulting unidirectional pulses would be of the opposite sign, but the operation would be otherwise .the same.

In order to keep the system stable when no reflected pulses are being received, it is preferable to feed a weak synchronising pulse from the generator 2 to the generator i0 so that the curves e and f in Fig. 6 will cancel each other and produce no pulses g. It can be seen from Fig. 6 wave form a, .that when t is a little less than T, the upper portion of the pulses g are long, and if the device i3 be arranged to shift these pulses upfwards until the lower portions coincide with the zero axis, then a large effective voltage will be applied to the meter il. If t is a little greater than T, then'this voltage will be very small, b ecausethen the upper portions of the pulse g will be small. This affords a means of giving an alarm when the obstacle comes within range, since the sudden change from a small to a large voltage at the terminals of the meter Il when time t becomes lust less than T can be made to perform some function such as the operation of a marginal :relay 28 which closes an alarm circuit. The synchronlsing pulse should of course be sufliciently weak to be overl-ruled when any reflected pulses are received.

The meter Il may clearly be calibrated in terms oi' the distance of the object and this calibration will be independent of the amplitude of the reilected pulses. In case the amplitude of the savv-A 4 tooth oscillations should not be maintained suiilciently constant', the calibration would be a'ected, but this can tude limiting arrangements for the ilnal pulses a so that their amplitude would become independent. of that of the oscillations. limiter ily of known kind is therefore inserted between the rectifier il and the voltmeter il. The measurements depends on the duration of the g pulses which in turn depends on the timing o! ythe fly-back stroke -and not on the actual amplitude of the saw-tooth waves.

- It will be apparent that if the instruments 8 or I4 are suitably calibrated they will give an immediate indication of the distance of the object as soon as it comes'within range. No manual ad- Justinents are required. and theobject is kept continuously under observation. Also, since it'is easy to provide alarm arrangements in the manner explained, the constant presence of an operator is not necessary.

Although the invention 'has been described in terms of particular embodiments, it is not intended to be limited thereto, and various modifications will occur to those skilled in .the art. 1

be 'overcome by the use offamplif What is claimed is:

1. System for measuring the distance of an object comprising a sawtooth wave oscillator having a normal operative period, a pulse generator controlled by said saw-tooth wave oscillator for generating electrical pulses, a radio transmitter, means for modulating the output of said radio transmitter by .said electrical pulses, means for receiving and detecting the pulse modulated waves after reflection by said object whereby electrical pulses are obtained, means for applying said received electrical pulses to said saw-tooth wave oscillator to initiate the ily-back stroke thereof prior to the normal termination of said period and thereby to control its frequency, an indicating device, means for deriving from an output of said saw-tooth wave oscillator s. voltage proportional to the frequency of the resultant 'generated sawtooth waves, and means for applying said voltage to said indicating device.

2. A system for measuring the distance of an object according to claim 1, wherein said sawtooth wave oscillator is adjusted to have a minimum frequency such that the period of a sawtooth wave cycle is at least equal to twice the time necessary for a pulse modulated wave to reach the most distant object which it is desired to locate.

3. A system for. measuring the distance of an dbject according to claim l, wherein means is provided for applying a blessing voltage proportional to the resultant frequency ofthe generated saw-tooth waves to said sawtooth wave oscillator.

4. A system for measuring the distance of an object comprising a saw-tooth wave oscillator having a normal operative period, a pulse generator controlled by the output of said oscillator, means for transmitting pulses derived from said pulse generator, means for receiving said pulses after reflection by an object, means for applying said received pulses to said oscillator prior to x the normal termination of said period to syn- An amplitude chronize said oscillator at a frequency which is 'inversely proportionalto the distance of the obiect, and means for indicating the distance of said object comprising means for measuring a characteristic of said oscillator.

PRAFULLA KUMAR CHA'I'IERJEA. LESLIE WIIFRED HOUGHTON.

REFERENCES CITED The following references are lof record in the nie of this patent:

Great Britain JulyV 26, 1937 

